1. Field of the Invention
The present invention relates to a ball screw used in a feed system of machine tools, general industrial machinery and the like. In particular, the invention relates to a ball screw suitable for high-speed feed motion.
2. Description of the Related Art
In general, a ball screw comprises a screw shaft having a thread groove on outer peripheral surface, a ball nut having a thread groove on inner peripheral surface opposed to the thread groove of the screw shaft and a plurality of balls to be inserted between the two thread grooves and recirculated along a ball circulation passage provided in the ball nut. So as to form ball circulation passage, return tube type, insert piece type, end cap type or guide plate type is adopted. In the ball screw used in the application for the positioning with high accuracy, a ball screw is held under a preload so that the axial clearance in the ball screw is eliminated and the rigidity of the ball screw may become larger with the lesser elastic displacement under the axial load. In addition, the two thread grooves are formed generally in form of gothic arc. The methods to apply preloading are roughly divided to: fixed position preloading and fixed pressure preloading (to apply tensile preloading with a spring between two ball nuts). As the fixed position pre-loading, over-sized ball preloading method or lead-shift preloading method is adopted in case of single nut. In case of double-nut, tensile preloading method to insert a thick spacer to match the preloading amount between the two ball nuts or compression preloading method to insert a thin spacer to match the preloading amount is adopted. (For instance, see: Minoru Izawa: xe2x80x9cApplied Technique of Ball Screwxe2x80x9d, 1st edition, published by Kogyo Chosakai, Ltd. (May 20, 1993), pp.18-21 and pp.49-51.)
In such a type of ball screw, chromium-molybdenum steel SCM415H or SCM420 H (JIS G 4105) is used as the material for the screw shaft and the ball nut. Surface hardness is held in HRC 58-62 (Hv 650-740) by carburizing and quenching and by tempering. As the ball to bear the axial load, steel balls for ball bearings (JIS B 1501) made of high carbon chromium bearing steel SUJ2 (JIS G 4805) is used. (Hereinafter, the steel balls for ball bearings is referred as xe2x80x9csteel ballsxe2x80x9d.) When preloading or pressurization is applied on the ball screw, spacer steel balls with diameter of several tens of xcexcm smaller than the diameter of steel balls are often disposed at a rate of every one ball, every two balls, or every three balls with the purpose of suppressing the increase of friction torque caused by jamming of steel balls against each other. Also, the screw shaft made of carbon steel AISI 4150H or S55C (JIS G 4051) and processed by induction hardening is often used.
In the ball screw with xe2x80x9conly steel ballsxe2x80x9d inserted in it, there have been problems as given below when high-speed feeding is performed at 60 m/min. or more, for instance.
{circle around (1)} Heat generation is increased, and this induces elongation of the screw shaft due to temperature increase. The elongation of the screw shaft leads to poor positioning accuracy.
{circle around (2)} When preloading is applied on the ball screw, the pre-load value increases due to thermal expansion of the steel balls or centrifugal force acting upon the steel balls when the screw shaft is rotated at high speed. The increase of the preload value leads to further increase of heat generation. That is, elongation of the screw shaft is increased, and this results in still poorer positioning accuracy.
{circle around (3)} Noise during ball screw operation is increased.
{circle around (4)} In case of the return tube type ball screw:
(i) Balls (steel balls or ceramic balls as to be described later) are separated from thread grooves on the screw shaft and these balls hit the tongue of the return tube when the balls enter the return tube. The increased hitting force of steel balls may repeatedly exerts action on the tongue, and the tongue may be damaged in many cases. (For instance, see xe2x80x9cApplied Technique of Ball Screwxe2x80x9d, ibid., pp.117-121.)
(ii) In the ball screw, a certain fluctuation or variation occurs in the circulating movement of balls due to deviation caused in fabrication and assembling stages. For this reason, the balls discharged from the return tube hit a portion near the land (cylindrical outer periphery of the screw shaft) of the thread groove of the screw shaft, and the balls are then caught in the thread grooves. The increased collision force of the steel balls repeatedly exerts action on a portion near the land and this results in damage on the thread grooves. As a result, practical service life of the ball screw is shortened.
As one of the means to solve the above problems, ceramic balls made of silicon nitride (Si3N4) with lower density and lower linear expansion coefficient than steel balls as shown in Table 1 (hereinafter referred as xe2x80x9cceramic ballsxe2x80x9d) are used instead of the steel balls.
However, in the ball screw with xe2x80x9conly ceramic ballsxe2x80x9d inserted in it, there have been the following problems:
{circle around (1)} When preloading or pressurization is applied on the ball screw and high-speed feeding is performed, temperature increase is relatively lower compared with the case where only the steel balls are inserted. However, axial clearance is apt to occur and then preload is lost, which will result in poorer positioning accuracy. The reason to cause the axial clearance is that the diameter of ceramic ball is smaller compared with sizes of the screw shaft and the ball nut made of steel (linear expansion coefficient: 11.9xc3x9710xe2x88x926/xc2x0C.), and linear expansion coefficient is also lower. As a result, it is more likely to be subjected to the influence of temperature increase.
{circle around (2)} Compared with the ball screw with only the steel balls inserted, the screw shaft has generally shorter service life. This is attributed to the fact that, if steel ball and ceramic ball have the same diameter, when the same axial load (i.e. the same magnitude of rolling element load) is applied, contact surface area of the ceramic balls and the two thread grooves is smaller than that of the steel balls. As a result, high contact stress is caused on contact surface, and this leads to shorter service life of the thread grooves (according to the elastic contact theory by Hertz).
{circle around (3)} When a rotating screw shaft is stopped and is rotated in reversed direction, shock load is applied on contact surface of balls (steel balls or ceramic balls) and the two thread grooves. (The higher the rotating speed of the screw shaft is, the higher the shock load is.) Ceramic balls are harder than steel balls and have higher modulus of longitudinal elasticity than steel balls (See Table 1 above), and indentation is more likely to occur on the thread grooves due to the shock load. As a result, noise during ball screw operation is increased, and service life of the thread grooves is decreased due to indentation. Thus, ceramic balls are inferior to steel balls in the shock resistance of the thread grooves.
{circle around (4)} Ceramic balls are very expensive in cost compared with steel balls. For instance, in case of a ball with nominal diameter of xc2xc inch (6.35 mm), market price of steel ball is about 0.8 yen/piece, while that of ceramic ball is 20 yen or more/piece.
It is an object of the present invention to provide a ball screw, by which it is possible to overcome the above problems in the conventional type ball screw with xe2x80x9conly steel ballsxe2x80x9d or xe2x80x9conly ceramic ballsxe2x80x9d inserted in it and which is suitable for the application of high-speed feed motion.
The present invention provides a ball screw, which comprises a screw shaft made of steel and having a thread groove on outer peripheral surface, a ball nut made of steel and having a thread groove on inner peripheral surface opposed to the thread groove of the screw shaft and a plurality of balls inserted between the two thread grooves and to be recirculated along a ball circulation passage provided in the ball nut, whereby the steel balls made of bearing steel and the ceramic balls made of silicon nitride are disposed alternately at a given ratio as the balls, and diameter of the ceramic balls is set to a value smaller than diameter of the steel balls so that contact stress acted on contact surface of the ceramic balls and the two thread grooves will be equal to contact stress acted on contact surface of the steel balls and the two thread grooves.
According to the present invention, when high-speed feeding is performed:
(1) In comparison with the conventional type ball screw with xe2x80x9conly steel ballsxe2x80x9d inserted:
{circle around (1)} Heat generation is low, and elongation of the screw shaft due to temperature increase can be reduced. Also, when preloading is applied on the ball screw, it is possible to suppress the increase of preload value caused by temperature increase due to high-speed rotation of the screw shaft. Therefore, positioning accuracy as required can be maintained.
{circle around (2)} Noise during ball screw operation can be reduced.
{circle around (3)} In case of a return tube type ball screw, the probability to damage the tongue of the return tube can be reduced. Also, damage on the thread groove of the screw shaft can be minimized.
(2) In comparison with the conventional type ball screw with xe2x80x9conly ceramic ballsxe2x80x9d inserted:
{circle around (1)} When preloading is applied on the ball screw, even when preload is lost as result of occurrence of axial clearance between the ceramic balls and the two thread grooves due to temperature increase, the preload between the steel ball and the two thread groove is not lost. As a result, positioning accuracy as required can be maintained. Also, in this case, the ceramic balls serve as spacer balls, and this contributes to the improvement of workability during high-speed feeding. When the insertion ratio of the steel balls to the ceramic balls is set to 1:1, the axial load to be born is decreased to one-half. Thus, it is preferable to set the insertion ratio to 2:1 or 3:1.
{circle around (2)} Service life will be extended. That is, it will be as long as the service life of the ball screw with xe2x80x9conly steel ballsxe2x80x9d inserted.
{circle around (3)} ceramic balls and thread grooves come closer to each other by elastic approach after elastic approach occurs between the steel balls and the thread grooves. Thus, even when shock load is applied, indentation hardly occurs on the thread grooves.
{circle around (4)} Production costs may be suppressed.